About 5 to 10 percent of American children are diagnosed as dyslexic. Historically, the label has been assigned to kids who are bright, even verbally articulate, but who struggle with reading — in short, whose high IQs mismatch their low reading scores. On the other hand, reading troubles in children with low IQs have traditionally been considered a byproduct of their general cognitive limitations, not a reading disorder in particular.

Now, a new brain-imaging study challenges this understanding of dyslexia. “We found that children who are poor readers have the same brain difficulty in processing the sounds of language whether they have a high or low IQ,” says John D. E. Gabrieli, MIT’s Grover Hermann Professor of Health Sciences and Technology and Cognitive Neuroscience, who performed the study with Fumiko Hoeft and colleagues at the Stanford University School of Medicine; Charles Hulme at York University in the U.K.; and Susan Whitfield-Gabrieli, also at MIT. “Reading difficulty is independent of other cognitive abilities.”